I. Field of the Invention
This invention relates generally to devices for controlling the rate at which a movable object is allowed to move relative to a fixed object, and more particularly to an improved design for a hydraulic dampener that is positionable between the movable object and the fixed object.
II. Discussion of the Prior Art
A variety of hydraulic dampening devices have been used in the past to limit the speed with which an object acted upon by an applied force will move relative to a fixed body. For example, in U.S. Pat. No. 5,297,912, assigned to applicant, there is described a ladder rack assembly that is adapted to be mounted to the roof of a van or other type vehicle for transporting ladders on the rack. A four-bar linkage assembly that is adapted to be actuated by the rotation of an elongated handle allows the ladders to be transported from a first position on and parallel to the top of the vehicle to a second position along side the vehicle where they can be readily removed from the rack and carried away by a workman. As the weight of the ladder is brought over center on the rack, gravity suddenly, takes over which would cause the ladder on the rack to drop suddenly were it not for the inclusion of hydraulic dampeners to slow down the movement of the ladders relative to the stationary vehicle.
Hydraulic dampeners of the type described are also often found on machine tools, such as a vertical/horizontal band saws. In this application, a hydraulic dampener can be used to control the rate at which the assembly carrying the orbiting saw blade will move through the workpiece as cutting takes place.
To achieve smooth and effective control, it is imperative that the speed controlling hydraulic damper not leak hydraulic fluid because when that hydraulic fluid is replaced by a compressible medium (air) erratic movement takes place. Rather than a smooth unidirectional descent, the movable object may bounce or oscillate. Ultimately, seal failure can lead to an uncontrolled descent, a situation to be avoided.
To avoid loss of hydraulic fluid and attendant air entry into the cylinder of the hydraulic dampener, it is required that there be an effective seal between the piston rod and the cylinder at the point where the cylinder rod exits the cylinder.
In prior art speed controlling hydraulic dampeners, as the piston rod is being drawn out from the cylinder, a vacuum is created because the volume in the cylinder formerly occupied by the piston rod is reduced. This, too, results in the introduction of air and a spongy performance when the piston is again driven back into the cylinder.
It is accordingly a principal object of the present invention to provide an improved speed controlling hydraulic dampener for controlling the movement of a fixed object relative to a stationary object.
Another object of the present invention is to provide a speed controlling hydraulic dampener having improved sealing structures for precluding the entry of air into the cylinder during actuations thereof.
Another object of the invention is to provide a speed controlling hydraulic dampener having means for compensating for the reduction in volume of hydraulic fluid in the cylinder as the cylinder rod is being drawn out from the cylinder to prevent the formation of a vacuum within the cylinder.